PROJECT SUIVIMARY (See Instructions): The progression of a primary mammary epithelial cell to the malignant phenotype is thought to involve multiple genetic events including the activation of dominant acting oncogenes and the loss of specific tumor suppressor genes. Of relevance to this proposal is the observation that activation of certain tyrosine kinases has been implicated in the malignant progression of a significant proportion of human breast cancers. Studies by many laboratories suggest that activation ofthe PI-3K signaling cascade plays a critical role in mammary tumor progression. The principle objective of this proposal is to elucidate the relative contribution of the EGFR family in activation of the PI-3K signaling cascade. To accomplish this objective, we plan to exploit two well characterized transgenic mouse models expressing either ErbB2 under the transcriptional control of the MMTV promoter (NDL) or under its own transcriptional control (ErbB2KI). In particular we plan to assess the relative contribution of ErbBS and its coupled downstream signaling molecules including PI-3K, Aktl and Akt2. Finally, we also will examine the in vivo role of Rab25 and its effector Rab coupling protein (RCP) in ErbB2 mammary tumor progression